The present invention relates to systems for supplying air to start an engine and fresh air to a cabin, such as in aircraft. More specifically, the present invention relates to an integrated bleed air and engine starting system that minimizes fuel penalties associated with the use of bleed air.
Efficiency in aircraft design remains an ever-present concern. Yet, future aircraft designs remain focused on reducing unit costs and operating costs. The design trend is to integrate system functions to reduce duplicate components to thereby reduce the unit cost. An approach to reducing operating costs is to lower the fuel consumption by designing a higher efficiency system.
In terms of operating efficiency, anti-ice systems and environmental control systems of aircraft typically operate with bleed air at intermediate or high pressures from gas turbine engines. But utilizing bleed air to operate these systems and their components results in operating penalties or, in other words, reduced engine efficiency. In particular, the penalty is increased fuel consumption. For instance, bleed air taken from an engine compressor is usually cooled and the pressure regulated before its ultimate use. Typically, engine fan air or ram air is used to cool the bleed air through a heat exchanger, which will have a negative impact to the engine and aircraft performance. The heat exchanger imposes a weight penalty to the aircraft. The bleed air taken from the engine for environmental control system (ECS) usage usually has a pressure higher than what the ECS needs. Thus, the pressure is regulated in a pressure regulator and throttled at a flow control valve to meet the ECS demand. Throttling the bleed pressure, however, means a waste of energy and imposes a fuel penalty to the aircraft.
A past attempt to lower the unit cost by integrating engine starting and thermal management is found in U.S. Pat. No. 5,363,641 wherein a starter compressor and a starter turbine are linked through a shaft to an engine. An auxiliary power unit provides air to the starter compressor which, in turn, provides compressed air to an auxiliary burner during a start mode or a heat exchanger during an operating mode. In the start mode, fuel is also fed to the auxiliary burner for combustion, with the combustion products then being flowed to the starter turbine. As the starter turbine accelerates, the starter compressor, in turn, accelerates. The starter compressor then accelerates the shaft to a high compressor in the engine until the engine becomes self-sustaining. In the operating mode, the shaft between the starter compressor and the engine are disengaged via a clutch. The compressed air from the starter compressor is flowed into a heat exchanger. From the heat exchanger, the air moves to the starter turbine, expanded, and then flowed to cool engine components. A disadvantage to this design, however, includes the fact that the turbine discharge air cannot be used for passenger breathing because of contamination during the starting mode.
In U.S. Pat. Nos. 5,143,329 and 5,125,597, during ground start operation of one engine, a starting turbine receives compressed air from a starting air supply such as bleed air from another engine and discharges the air overboard. The starting turbine consequently cranks a high pressure turbine shaft within the engine until the engine can continue operation off of an engine compressor and without assistance from the starter turbine, although the starter turbine remains connected to the turbine shaft. During flight, a primary heat exchanger of an ECS receives an outlet flow from the starting turbine. The flow from the primary heat exchanger moves through a compressor, a secondary heat exchanger, and then an ECS turbine. From the ECS turbine, the air can be used to cool a cabin. A drawback of this design is that the pressure of the compressed boundary layer flow is too low for ECS operation and, thus, does not offer bleed air reduction for fuel savings.
Boundary layer bleed air is used in U.S. Pat. No. 5,136,837; to feed a compressor. During cruise operation, the compressor provides compressed air to a turbine and the outlet from the turbine is then used for cooling. During start-up, air to the turbine can be supplied from a ground supply or auxiliary power unit. The turbine outlet flow can then pass into the engine. Limitations in this design, however, include the fact that the turbine cooling flow is unmixed and is supplied for engine cowl cooling. Also, there is no mention in reducing the bleed air penalty associated with cabin fresh air supply.
Other related disclosures include U.S. Pat. Nos. 5,490,645; 5,414,992; 4,916,893; and 4,684,081.
As can be seen, there is a need for an improved integrated system for supplying bleed air and starting an engine. Also needed is a system that supplies air not only to start an engine but also to supply air to an environmental control system. Another need is for a system that can start an engine while minimizing associated fuel penalties. In that latter regard, there is a need for an engine starting system that minimizes fuel penalties by maximizing the use of existing aircraft components. A further need is for a system that can multiply an air flow to supply an environmental control system, thereby lowering flow mixing temperatures and reducing a high stage bleed penalty. A particular need is for an integrated system of bleed air supply and engine starting.